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With dozens of scenarios, control of the Senate is a toss-up

As political nail-biters go, the most interesting battle in the 2012 election does not involve anyone named Obama or Romney. While the general election is going to be decided in six or seven swing states that have a reasonable likelihood of going for either presidential candidate, control of the Senate rests in a much more complex tangle of eight or nine states. Right now, our model of polls, prediction markets and historical data suggests that Democrats have a 48.7 percent likelihood of retaining control of the upper body.

The Democratic Party currently has 53 senators in its caucus to the Republican Party's 47 senators. But the way the cards fell with the 33 seats up for re-election this year was not kind to the incumbent party. Democrats are defending 23 seats while the Republicans are defending only 10 seats in this year's election. Stated another way, the Democrats have just 30 seats confirmed for the next session to the Republicans' 37 seats among the 67 total that aren't up for re-election for two or four more years.

Since the vice president casts a tie-breaking vote in the Senate, the Democrats must win 20 seats and the presidency or at least 21 seats to retain the Senate. The Republicans must win 13 seats and the presidency or at least 14 seats to take over the Senate.

Of course, not all of the 33 seats up for election are really in play. After narrowly surviving the primary, Republican Sen. Orrin Hatch is virtually certain to win re-election in Utah, and Democrat Kirsten Gillibrand will safely win a new term in New York. Here the Democrats pick up a small amount of ground, with 10 seats virtually certain to go to them, to the Republicans' five.

Both parties have five more seats that are at least 80 percent likely to go to them. As we've said time and again, aberrations are very possible even in these seemingly secure races. In fact, we expect one of these seats to end up with the other party.

That leaves eight seats that are in the highly uncertain category of between 20 to 80 percent likely victory for the parties.

Three of these seats are leaning Republican: Nevada has a 74.8 percent likelihood of going Republican, Montana 62.5 percent and Wisconsin 57.0.

Wisconsin is holding its primary tomorrow, and it's a tight race on the Republican side. The Democratic nominee, Tammy Baldwin, will be slightly favored over Eric Hovde or Mark Neumann. Tommy Thompson, should he win the primary, will be slightly favored.

Four of these states are leaning Democratic: Virginia has a 50.1 percent likelihood of going Democratic, Massachusetts 62.8 percent, Florida 69.5 percent and Ohio 73.4 percent. Virginia is open due to a retiring Democratic senator, but features a very compelling race between former Gov. Tim Kaine (Democrat) and former Sen. George Allen (Republican) who lost this same seat six years ago in the tight race that flipped control of the Senate to the Democratic Party.

And Massachusetts is the most anticipated race of the year, with incumbent Republican Scott Brown defending his seat against Democrat Elizabeth Warren. Sitting Democratic senators are defending both Florida and Ohio.

The final seat is Maine, which is 80.0 percent likely to go to Angus King, who is independent. He refuses to state which party he will caucus with, but it is more likely that it will be Democratic, as he has stated his support for President Barack Obama.

If all of the races go in the direction they currently point, then the Republicans will have 50 seats, the Democrats 49 seats, and Angus King will win Maine. That leaves some interesting scenarios. First, the Republicans control the Senate if they get either King's vote or the vice-president's vote if Romney wins the presidency, which he's about 40 percent likely to do at the moment. The Democrats control the Senate if they get both King's vote and the vice-president's vote.

David Rothschild is an economist. He has a Ph.D. in applied economics from the Wharton School of Business at the University of Pennsylvania. Follow him on Twitter @DavMicRot and email him at thesignal@yahoo-inc.com.